1. Field of the Invention
The present invention relates generally to a process, apparatus and method to facilitate gasification of various carbonaceous fuels including low reactivity coals in fluidized beds. The invention can handle fuels containing higher percentages of ash and fouling components such as iron, sodium or potassium. More particularly, the present invention relates to a second stage gasification unit in a staged fluidized bed gasification process, which process can gasify nearly all carbonaceous materials and achieve high carbon conversions and produce a tar free and dust free synthesis gas (syngas) with lower concentrations of polluting components than is capable with the conventional process that includes a multistage fluidized bed syngas cooler, a low temperature cyclone and a barrier filter.
2. Description of Related Art
Gasification technology is an effective process to effectively convert carbonaceous resources such as different grades of coal, petroleum residues and cokes into syngas. The syngas can be used in many applications, including downstream chemical synthesis, generating power via its combustion in an IGCC processes producing nearly zero emissions of pollutants as well as effectively capturing carbon dioxide, and/or producing hydrogen by shifting it for utilization in oil refining and coal liquefaction processes.
Conventional gasification processes have substantial limitations in processing wide varieties of carbonaceous resources. For example, entrained flow gasifiers are not suitable economically to process low reactivity fuels that have characteristics such as high ash content or high ash fusion temperatures. Fluidized bed gasifiers are normally designed for low ranks such as lignite and sub-bituminous coals due to their high reactivities. They have difficulties in gasifying high rank coals such as bituminous and anthracite with high carbon conversions. Further, the fluidized bed gasifiers can generate tar and have high methane content in the syngas, both of which significantly decreases syngas utilization to synthesize chemical products by downstream processes. Both the low carbon conversion and formation of tar and methane in the syngas are highly negative performance factors for a fluidized bed gasifier when gasifying high rank coals and other low reactivity fuels.
The fundamental reason for the limitations of fluidized bed gasifiers is that the process must avoid clinker and agglomerate (collectively sometimes referred to herein as “clinkers”) formation in the gasifier. Once formed, clinkers can debilitate the gasifier in a very short time and lead to a long outage. To avoid clinker formation, the gasifier operating temperature must be substantially lower than the ash initial deformation temperature because the particle surface temperature is substantially higher than the overall bulk temperature due to partial oxidation on the char particle surface. Making things worse is the fact that the bed carbon content is much higher when gasifying high rank coals due to the low reactivity of the coal. The oxidant (air, enriched air or oxygen) injected into the gasifier will be immediately consumed in a small volume space and create local hot spots favoring clinker formation. Therefore, when a fluidized bed gasifier is used to gasify a bituminous coal, the operating temperature is much lower than the ash initial deformation temperature. Such low operating temperatures in conjunction with the coal's low reactivity leads to lower carbon conversion, tar formation and higher methane content in the syngas.
One way to avoid the conventional limitations of the fluidized bed gasifier is by utilizing a stage-wise approach. For example, the Ebara Process disclosed in U.S. Pat. No. 6,063,355 teaches a two-stage approach to generate syngas from various types of waste refuse. The Ebara Process has a first stage fluidized bed gasifier operating at much lower temperature than a second stage conventional entrained flow gasifier with a burner, and has a relatively low capacity. The Ebara Process, with relatively low capacities in the 100 to 300 tons/day range, has a first stage fluidized bed gasifier operating at around 580° C., and the second stage entrained flow gasifier operating at around 1300° C. In this manner, the stage-wise Ebara Process essentially gasifies waste fuel at the low first stage temperature, which minimizes fuel feed related problems and provides favorable conditions for recovering useful wastes from the bottom of the fluidized bed gasifier. In the second stage, the Ebara Process further conditions the unconverted char, tar and organics in the syngas from the first stage.
However, the Ebara Process cannot efficiently process bituminous coals and other low reactivity fuels with high ash content just as other entrained flow gasifiers, since the second stage gasifier operates with the same disadvantages described with the conventional entrained flow gasifiers—it requires much higher temperatures and oxygen consumption, and thus provides a shorter refractory lifespan—which are in addition to the well-known grey water and black water problems.
Moreover, it is well-known that the mixing effect is very poor in entrained flow gasifiers such as in the second stage gasifier used in the Ebara Process, especially when the gas phase carries quite a low concentration of dust. One example of poor mixing and reaction extent is the freeboard region of a fluidized bed gasifier. In spite of relatively long residence times, limited degrees of chemical reactions occur in the freeboard region of the fluidized bed reactor. A majority of reactions occur in the dense fluidized bed region of the reactor. The same can be said for the entrained flow gasifier. With the increase in size of the gasifier for larger throughputs, poor mixing between the gas and solids results, with progressive deterioration in carbon conversion.
The effect of poor mixing is much more prominent with the second stage gasifier of Ebara Process than with a conventional entrained flow gasifier in which both coal and oxygen are concentrated in the highest temperature burner tip region. Only a small fraction of fuel particles escapes the flame region, and thus never gets another chance for conversion. In contrast, the char particles and the tar vapors tend to accumulate in the relatively lower temperature wall region due to the poor mixing of the swirling-type second stage entrained flow gasifier of the Ebara Process. Abnormally higher temperature at the oxygen injection region can increase the overall temperature in the second stage gasifier of the Ebara Process at the expense of increased oxygen consumption and shorter refractory lifespan.
Other methods to avoid the conventional limitations of the fluidized bed gasifier have been proposed that essentially act as a second stage gasifier. One such example is disclosed in U.S. Pat. No. 4,412,848 that teaches a method of injecting oxygen at a lower inlet section of a second stage cooler, where the operating temperature is about 400-500° C. U.S. Pat. No. 4,412,848 purports to reduce the tar deposition on the heat transfer media particle surfaces. Except for the inlet section, the overall cooler bed temperature is only about 250-300° C., which is below the ignition temperature of syngas components (carbon monoxide, hydrogen and others), raising a significant safety concern. Further, such a low temperature partial oxidation does not appreciably improve carbon conversion. For low reactivity coals, the carbon conversion in the first stage gasifier, if operated at about 1000° C. to limit/avoid clinker formation, will be less than 80%, and therefore the second stage must operate at much higher temperatures to increase the carbon conversion.
Another attempt to improve conversion of char particles and tar in the syngas stream is to directly inject oxygen to the freeboard region just above the fluidized bed gasifier, see, for example, http://www.fischer-tropsch.org/primary_documents/gvt_reports/BIOS/333/BIOS—333_toc.htm, as has been practiced in the known Winkler gasifier since 1930s. Since oxygen is directly injected into the gasifier to increase the freeboard temperature, the cost of implementation is low. However, overall mixing and temperature uniformity in the vessel with gas flows carrying only up to 40,000 part per million by weight (ppmw) dust is poor. In spite of improvements, the effect of oxygen injection to the Winkler gasifier freeboard region still results in low carbon conversions.
U.S. patent application Ser. No. 13/936,457, hereby incorporated by reference, discloses a two-stage gasification process to efficiently gasify high ash bituminous and semi-anthracite coals and other low reactivity fuels with over 95% carbon conversion and the generation of tar-free syngas for further processing. To convert carbon sources with low reactivity, the second stage fluidized bed gasification unit needs to operate at temperatures in the range of 1100-1400° C. At these high gasifier exit temperatures, costs associated with cooling the syngas effectively and efficiently become a challenge. U.S. patent application Ser. No. 14/010,381, hereby incorporated by reference, presents an apparatus and method to cool high temperature syngas in a multistage circulating fluidized bed syngas cooler.
The potential beneficial utilization of the combination of these two innovative systems, along with a conventional barrier filter, still poses challenges in configuring and operating a gasification process to gasify a wider variety of carbon sources.
To overcome the operability, efficiency and cost issues mentioned above, an integrated and staged gasification process that can gasify various carbonaceous materials is highly desirable. It is the intention of the present invention to provide for such an industrial need.